Quoted:Dennis2313, let break this down to two different working pressures.
First is the over all working pressure of the round itself. Since we are talking 308, then SAAMI PSI 62,000.
Faster burning powder will acheive this top over all pressure spike faster down the barrel than say slower burning powder, but both can produce a working pressure of 62K. Granted that the top of the pressure curve spike point may only be a inch or two difference down the barrel, It's this dwell/top of pressure curve spike location that plays back into how much the barrel gas port/gas system will see in pressure. I Don't have graph in front of me, but the difference in say 150 ball ammo, to Hornaday match ammo could be in the range of 10K or more at the gas port, although both are working at 62K over all working pressure.
If you take a look at this below linked topic, it will give you an idea of overall pressure, and pressures at the gas port. Keep in mind that the powder being used is standard burn rate, and if a slower burning pressure was being used, then the peak over all spike pressure would be closer to the gas port, increasing the pressures at the gas port as well.
http://www.ar15.com/forums/t_3_66/266108_How_it_Works__The_AR_gas_operation_and_how_everything_works_in_harmony_.htmlNow lets get down to the unlock.
B/C carrier mass, recoil spring tension, and the amount of gas that the gas system see's, all play the Math part in how the system works. As stated, the pulse of the gas system happens as the bullet passes the gas port, then this pressure starts to drop as soon as the bullet exits the barrel.
So at the bolt, we first have the gas system charge pushing the B/C back, which includes mass of the B/C, the mass of the buffer, and the spring tension. Here, we are looking for enough gas pressure force to full stroke the B/C all the way back, but at the same time since the bolt has to cam around to the unlocked position before the bolt lugs can clear the barrel extension lugs, enough gas pressure drop in the barrel bore before the bolt/barrel lugs clear each other so the spent case is not pressure welded to chamber so much that the bolt can not pull it out cleanly.
Again, increasing the mass of the B/C and buffer, and even adding buffer spring tension slows the initial movement of the carrier back at gas charge (micro seconds), which slows the unlock down just enough to allow more barrel residual pressure drop to happen before the bolt actual pulls the spent case out of the chamber.
So how does this all come together?
We have a round loaded for a over all 62K pressure.
Since we have a gas system, then where the gun powder peak burn dwell happens down the barrel bore, will dictate what amount that the gas ports sees. Since the rifle is based off standard burn rates, when you use slower burning powder to push heaver bullet down the barrel (while still staying in the 62K max pressure allowed), your going to have higher gas port pressures (peak dwell closer to the gas port). Since the math of how the rifle unlocks is gas pressure against Mass of the B/C and Buffer, and the tension of the recoils spring, increases these will slow down the bolt unlocking and begins to pull the spent case out of the camber (bore residual gas pressure drop) back into check.
As for why not just use the same powder burn rate for all loads, it does not work out this way. The heaver the bullet, the slower the powder burn needed to get the bullet up to top speeds while staying in the over all working pressures,which increases the gas port pressure. Same goes for lighter than normal bullet loads, with faster burning powder used, which means a decrease in gas system pressures instead (all the while both over all working pressures of both loads at 62K).
OK, thanks for the explanation... Most of it makes sense to me, as far
as the pressures, fast and slower burning powder, the peak pressures
and such. I'm still unclear on the actual bolt rotating to unlock. I
still can't get it into my head that any other force than just the gas
pressure will effect the rotation and unlocking of the bolt. In my
mind, nothing else happens until the bolt rotates which as far as I can
tell is only controlled by the gas. Unless the bolt lugs are held
against the barrel extension lugs tight enough to cause enough friction
and resistance to delay rotation. Is this it?? Is the bolt not free in
the barrel extension but actually held tightly against each other?
Nevermind....
I just studied how my bolt unlocks and I just realized that the bolt is
unlocking as the bolt carrier begins to move back and the cam pin is
actually what is causing the bolt to rotate. I was thinking that the bolt had to unlock first before the carrier would move, but now I see that it is a simultaneous action and the bolt carrier mass and spring pressure will certainly effect bolt unlocking speed.
It all makes sense to me now.
Thanks!